The invention relates to an abrasion-resistant, multilayer, biaxially oriented carrier film for magnetic information media, composed of a base layer A, comprising a thermoplastic polymer matrix made of essentially linear aromatic polyester, containing finely distributed solid particles to improve its surface roughness, which determines the slip properties of the film, and of at least one cover layer B, applied to one of the two surfaces of base layer A.
Biaxially oriented films of polyesters, especially polyethylene terephthalate, because of their superior properties such as tensile strength, tear resistance, modulus of elasticity, transparency, chemical resistance, thermal resistance and the like are used in large quantities in many technical areas, for example also as carriers for magnetic recording media.
However, the polyester films must have a specific combination of properties. Thus, for example polyester films to be processed into magnetic tapes to be used for audio, video, or computer technology must have a low coefficient of friction and a high abrasion resistance and must not be sensitive to deterioration of their electromagnetic transmission properties.
Methods are already known for improving the properties of polyester films by incorporating fine particles of an inert material into the films, for example by using one of the methods listed below:
(1) In general, the residue of metallic compounds used as re-esterification catalysts is precipitated in the system by special measures in the form of finely distributed particles (termed the "precipitation method"); and/or PA1 (2) microparticles of an inorganic or organic compound with different particles sizes are added from the outside (termed the "addition method"). PA1 (i) a polyester substrate layer having from 0.001 weight percent to 10 weight percent, based upon the weight of the polyester forming said substrate layer, of finely distributed solid particles having an average particle size of from 0.3 to 20 microns; and PA1 (ii) a homopolyester cover layer applied to at least one of the two surfaces of said polyester substrate, said homopolyester cover layer having a maximum thickness of 40% of the thickness of said polyester substrate, and having from 0 to 10 weight percent, based upon the weight of the homopolyester forming said homopolyester cover layer, of finely distributed solid particles having an average particle diameter of less than 0.3 micron. PA1 (i) separately plasticizing the polyester and homopolyester resins which are to form the layers of said film; PA1 (ii) combining said plasticized resins and extruding said resins in sheet-like form through a coextrusion nozzle; PA1 (iii) contacting the combined, multilayer extrudate upon a cooling roller, thereby cooling said extrudate in an unstretched state; PA1 (iv) biaxially orienting said cooled multilayer extrudate to form a biaxially oriented multilayer polyester film; PA1 (v) heat-fixing said biaxially oriented multilayer polyester film, PA1 (A) regulating the speed at which the plasticized polyester resins are permitted to pass to said co-extrusion nozzle, or PA1 (B) regulating the speeds of the extruders. PA1 1. The surface roughness remaining after application of layer B, said roughness being produced by the particle-conditioned surface structuring of layer A, which partially penetrates cover layer B or which in any case also produces a certain surface structuring of cover layer B, must be sufficiently large to produce frictional characteristics in the film which positively influence the winding of the film as well as the playing behavior of magnetic tapes manufactured from the film. PA1 2. The particles must be sufficiently far removed from the free surface of cover layer B to prevent the particles from tearing free, thereby preventing abrasion.
Single-layer film containing finely distributed organic particles are described for example in German Offenlegungschrift No. 30 19 073. Films which contain inorganic particles to improve their slip properties are described in German Auslegeschrift No. 21 33 895 and German Patent 26 47 713. These films are single-layer films and therefore have on both surfaces a surface roughness which depends on the number of particles or the diameter of the particles added in each particular case.
To reduce the surface roughness of the surfaces of films intended for receiving a magnetic coating, multilayer films are also manufactured which are composed of a first layer of thermoplastic polymer without particles added, which has a comparatively smooth surface, and a second, thinner layer, provided with particles in the usual fashion.
It has been found that the greatest disadvantage of particles incorporated into conventional polyester films for the purpose of improving the slip of the film lies in their lack of affinity for the polymer material and their tendency to form agglomerates. For this reason, the friction between such conventional films with one another or between such a film or films, which causes for example the formation of white dust or deposits on the films used for magnetic tapes. These deposits are extremely undesirable since they settle in the equipment used to play the magnetic tapes, causing damage therein.
If the particles which are normally distributed uniformly in the polymers forming the individual layers are deposited very close to the surface of the layer, friction during winding or rewinding of tapes that are already coated on one side with a magnetic coating or of films that are not yet coated, the extremely thin polymer layer flakes covering the particles can be loosened, releasing the particles embedded beneath. The extremely thin polymer layer flakes, however, adhere very strongly to the film as a result of an electrostatic charge and, since the tape is wound, can be transferred to the magnetic coating, thereby causing undesirable disturbing effects such as weakening of the signals, e.g. in the form of so-called "drop outs". In addition, the released solid particles penetrate the magnetic coating causing deformations and eventually signal losses or distortions as well. These disturbing effects, summarized under the term "abrasion behavior" in the least favorable case render the magnetic tapes produced from these carrier materials unusable for a designated application such as video, audio, or computer tapes. These considerations also apply to flexible disks (floppy disks).
Therefore, the object of the present invention is to proceed on the basis of the demonstrated state of the art to create a carrier film for magnetic information media, which (1) has a low surface roughness on the surface designated to receive the magnetic coating, (2) has a high surface roughness which determines the slip properties on the surface opposite the magnetic coating, (3) has sufficiently high strength in the lengthwise and transverse directions to allow manufacture of extremely thin tapes with extremely high signal density, and (4) has excellent abrasion behavior, so that even when the film is subjected to considerable frictional stress, no deposits can be seen on its surface.